In recent years, as the packing density and operation speed of semiconductor integrated circuits increase, the pattern line width of the integrated circuits is decreased, and a higher-performance semiconductor fabricating method is sought. Accordingly, as an exposure apparatus used for resist pattern formation in the lithography process of the semiconductor fabrication process, an exposure apparatus utilizing a short exposure wavelength of, e.g., extreme ultraviolet rays such as a KrF laser (248 nm), an ArF laser (193 nm), or an F2 laser (157 nm), or of X-rays (0.2 to 1.5 nm) has been developed.
In exposure using X-rays among these light beams, a proximity exposure method of moving an X-ray mask having a desired pattern to be close to a resist-coated wafer, and irradiating the wafer with X-rays through the X-ray mask, so that the mask pattern is transferred onto the wafer, has been developed.
In order to obtain high-intensity X-rays, an exposure method using synchrotron radiation is proposed. The technique has developed to such a degree that a pattern of 100 nm or less can be transferred. A synchrotron radiation source requires large-scale facilities. A profit cannot be expected unless device fabrication is performed by connecting ten or more exposure apparatuses to one light source. Hence, an exposure apparatus using a synchrotron radiation source is a system that is suitable for application to a highly demanded device such as a semiconductor memory.
In recent years, a device using GaAs has been input into practical use as a communication device, and a laser decrease in line width is required. Communication devices are produced in an amount less than that of semiconductor memories, and many types of communication devices are produced in small amounts. When an X-ray exposure system using synchrotron radiation as the light source is introduced to the fabrication of communication devices, it will probably make no profit. For this reason, an exposure apparatus using a compact X-ray source which generates high-intensity X-rays is used in actual communication device production. The light source ranges from one which is called a laser plasma beam source and generates a plasma by irradiating a target with a laser beam and uses X-rays generated by the plasma, to one which generates X-rays by generating a pinch plasma in a gas. These light sources are called point sources. According to a general arrangement, one exposure apparatus which transfers a pattern by aligning a mask and wafer is connected to one point source.
FIG. 6 schematically shows an X-ray exposure apparatus using a conventional point source.
Referring to FIG. 6, reference numeral 101 denotes an X-ray source unit for generating X-rays. In the X-ray source unit 101, a laser beam 121 is focused on a target 111 to irradiate it, in order to generate a plasma, thus generating X-rays 117. The X-rays 117 globally diverge from a light-emitting point 112. Some X-rays 117 are guided into a reduced-pressure He chamber 141 through an X-rays transmitting window 113.
A mask 131 has a transfer pattern. A wafer 133 coated with a photosensitive agent is positioned at a position with a small gap of about 10 μm from the membrane by an alignment unit (not shown). The wafer 133 is irradiated with the X-rays 117 emerging from the light-emitting point 112, so the pattern is transferred onto the wafer 133. The wafer 133 is sequentially stepped by a wafer stage 132 and is exposed successively. In some cases, a collimator is installed midway along the X-ray path between the light-emitting point 112 and mask 131 and focuses and collimates the X-rays.
The arrangement of the above conventional exposure apparatus will be described in further detail.
The conventional exposure apparatus is mainly comprised of the X-ray source unit (also to be referred to as a light source unit hereinafter) 101 and a main body 102. The light source unit 101 is set on the main body 102. The target 111 is arranged in the light source unit 101, and is irradiated with the laser beam 121 to generate a plasma, thereby generating the X-rays 117. The laser beam 121 is generated by a laser beam generating unit 122 separately installed on the floor, and is focused on the target 111 to be irradiated through a laser beam optical system (not shown).
The interior of the light source unit 101 is held at a vacuum, and the main body 102 is set in a reduced-pressure He atmosphere by the reduced-pressure He chamber 141. The light source unit 101 and the main body 102 are isolated from each other by the Be-made X-ray transmitting window 113 with a thickness of several μm, so that the vacuum atmosphere will not be spoiled. Beryllium has a high X-ray transmittance, but does not transmit He, so Be is used to form an X-ray transmitting window. A bellows A (denoted by reference numerals 116) is set between the light source unit 101 and reduced-pressure He chamber 141 to isolate them from the outside.
FIG. 7 shows the arrangement of the light source portion in detail.
The target 111 for generating the X-rays forms a tape, and is held as it is wound around a tape roll 119. The tape-like target 111 fed from the tape roll 119 is taken up by a takeup section 120. The focused laser beam 121 irradiates the target 111, extending between the tape roll 119 and takeup section 120, by pulse emission, and the X-rays 117 are radially generated from the light-emitting point 112 at each pulse. Every time the laser pulse is irradiated, the target 111 is gradually fed out and taken up by the takeup section 120. The target 111 is made of Cu or the like.
The main body 102 is set in the reduced-pressure He chamber 141, and is entirely maintained in the reduced-pressure He atmosphere by a helium atmosphere creating unit (not shown). This is because attenuation of the X-rays can be suppressed and a high heat transfer efficiency can be maintained if the atmosphere where the X-rays, as the exposure light, pass is set to a reduced-pressure He. The main body 102 is comprised of a mask stage (not shown) for holding and positioning the mask 131, the wafer stage 132 for holding, positioning, and stepping the wafer 133, a transfer system (not shown) for transferring the mask 131 and wafer 133, and a measurement system (not shown) for measuring the positions of the mask 131 and the wafer 133 relative to each other. The entire portion of the main body 102 is installed on the floor through vibration damping units 136. A stage surface plate 134 is set on the vibration damping units 136, and the wafer stage 132 moves on it, so that exposure is performed successively. A main body frame 137 is set on the stage surface plate 134, and supports the mask stage (not shown), the mask 131, and the like. The vibration damping units 136 prevent the positioning precisions of the mask 131 and wafer 133 that require precise positioning from being decreased by vibration from the floor, so the main body 102 maintains a constant posture.
Bellows B (denoted by reference numerals 142) are set between the reduced-pressure He chamber 141 and main body 102 so the reduced-pressure He atmosphere will not be spoiled when the posture of the main body 102 changes.
In the conventional apparatus arrangement described above, the light source unit 101 uses one type of target 111 as the X-ray generating source. The spectral intensity of the emitted X-rays depends on the type of the target 111. FIG. 8A shows an example of a transfer pattern intensity distribution on the wafer surface, which is obtained when a Line & Space pattern is exposed with the conventional exposure apparatus. In this example, the small gap distance (exposure gap) between the mask and wafer is 10 μm.
Ideally, the exposure apparatus resolves an image irrespective of the pattern, as shown in FIG. 8B. Factors that determine the image intensity distribution also include the material and thickness of the mask. As shown in FIG. 8A, with a 100-nm pattern, the intensity image is formed faithfully in accordance with the mask pattern. With a 70-nm Line & Space pattern, however, the image loses its shape, and the contrast decreases, so the image is not resolved. In other words, with the conventional X-ray generating source, since the exposure wavelength is a specific wavelength, depending on the mask pattern, the influence of diffraction becomes conspicuous, and sometimes, the image cannot be resolved. In this example, the pattern loses its shape and the contrast decreases. Depending on the pattern, a positional error may occur.
This problem may be avoided if the exposure gap is set to an appropriate amount. When the exposure gap changes, however, as the exposure light broadens, the positional error of the pattern to be transferred increases, and an overlaying error occurs. Therefore, this countermeasure cannot be used in practice.